You focus yourself to draw from your shadow as ink, with a Calligraphy brush of a medium or finer size you can write a message that will appear to be just a shadow from the surrounding area to the ordinary eye, you and any creatures you designate when you cast the spell, the writing appears normal, as written in your hand, and conveys whatever meaning you intended when you wrote the text. To all others.
The message cannot be written onto any reflective or polished surface and a creature with truesight can read the hidden message.